Chapter 22: Evolution by Natural Selection

Transcription

1 Evolution

2 Chapter 22: Evolution by Natural Selection Charles Darwin ( ) British naturalist o Proposed the idea of evolution by natural selection o Collected clear evidence to support his ideas Voyage of the HMS Beagle ( ) o Travels around the world o Makes many observations of natural world Main mission of the Beagle was to chart South American coastline. Galapagos o Of relatively recent volcanic origin most of animal species on the Galápagos live nowhere else in world, but they resemble species living on South American mainland. Darwin s Finches 13 species of finches on Galápagos Puzzling finding o Only 1 species of this bird on mainland of South America, 600 miles to east o All presumably originated from mainland Differences in beaks o Associated with eating different foods. o Adaptations to foods available on islands. Darwin s conclusions o When original South American finches reached islands, adapted to available food in different environments. Finches with beak differences that allowed them to o successfully feed o successfully compete o successfully reproduce pass successful traits onto their offspring 1

3 In Historical Context Western culture resisted evolutionary views of life Theories of geologic gradualism helped clear path for evolutionary biologists Lamarck placed fossils in an evolutionary context What did Darwin say? Variation o Every population of organisms includes differences between individuals. Where does variation come from? o Mutations are a source of new alleles. o Changes in nucleotide sequence of DNA. o Mutations of genes are rare & random. shot in the dark may be damaging = sickle cell disease may be silent introns redundancy of codons o Only mutations in gametes are passed on. Effect: More or less fit? o Mutations that alter protein structure enough to impact its function. More likely to be harmful than beneficial. Our genome is product of thousands of generations of selection. o Fuel for evolution Mutant allele may enable an organism to fit its environment better & increase reproductive success. Especially likely if environment is changing. 2

4 What did Darwin say? Over-production o Organisms reproduce more than the environment can support o Some offspring survive & some offspring don t survive Competition o for food, for mates, for nesting spots, to escape predators Natural selection Put together variation and competition and you get natural selection o Survival of the fittest o fittest are the ones that survive to reproduce Who is the fittest? o Traits fit the environment. o The environment can change. o Therefore who is fit changes. Peppered Moth (Why did the population change?) o early 1800s = pre-industrial England low pollution lichen growing on trees = light colored bark o late 1800s = industrial England factories = soot coated trees killed lichen = dark colored bark o mid 1900s = pollution controls clean air laws return of lichen = light colored bark o Industrial melanism Support for Darwin s ideas Fossil Record: change over time Biogeography: related organisms in similar range Comparative Anatomy Comparative Embryology Molecular Biology: measure of common ancestry Artificial Selection: induced evolution Fossil Record Layers of sedimentary rock contain fossils New layers cover older ones, creating a record over time Fossils within layers show that a succession of organisms have populated Earth throughout a long period of time. Acquired Traits By comparing current invertebrate species with fossil forms, Lamarck could see what appeared to be several lines of descent o chronological series of older to younger fossils leading to a modern species o proposed a mechanism to explain how specific adaptations evolve adaptation to the environment use & disuse causes modifications Acquired Characteristics are inherited evolution is influenced by need to survive 3

5 Biogeography Geographical distribution of species o Species living in the same region tend to be more closely related to each other o Species which look similar but are from different parts of the world are often not closely related Convergent Evolution o Evolving similar solutions to similar problems Analogous structures o Convergent evolution of structures similar functions similar external form different internal structure & development different origin no evolutionary relationship o Dolphins: aquatic mammal o Fish: aquatic vertebrate have adapted to life in the sea not closely related Comparative Anatomy Homology o Similarities in characteristics resulting from common ancestry. Forelimbs of human, cats, whales, & bats share same skeletal structures o similar structure o similar embryological development o different functions o evidence of common ancestor branched off from common 4-limbed ancestor Vestigial Organs Modern animals may have structures that serve little or no function o Remnants of structures that were functional in ancestral species o Evidence of change over time Some snakes & whales show remains of the pelvis & leg bones of walking ancestors Eyes on blind cave fish Human tail bone Hind leg bones on whale fossils. Why? 4

6 Comparative Embryology Similar embryological development in closely related species o All vertebrate embryos have a gill pouch at one stage of development Fish, frog, snake, birds, human, etc. Molecular Biology Comparing DNA & protein structure o universal genetic code! DNA & RNA o cytochrome C (respiration) protein structure o hemoglobin (gas exchange) protein structure Evolutionary relationships among species are documented in their DNA & proteins. Closely related species have sequences that are more similar than distantly related species. Artificial Selection Artificial breeding can use differences between individuals to create vastly different breeds or varieties. Natural selection in action Insecticide & drug resistance o Insecticide didn t kill all individuals o Resistant survivors reproduce o Resistance is inherited o Insecticide becomes less & less effective Theory of Evolution Evolution o Principle of evolution o Evolution is a fact o Observable & testable Natural selection o Widely accepted mechanism o Predictions have withstood thorough, continual testing by experiments & observations. Unity & Diversity Only evolution explains both the unity & diversity of life. By attributing the diversity of life to natural causes rather than to supernatural creation, Darwin gave biology a strong, scientific, testable foundation. 5

7 Chapter 23: Evolution of Populations Populations Evolve Natural selection acts on individuals o Differential survival survival of the fittest o Differential reproductive success bear more offspring Populations evolve o Populations of organisms change over time. o Traits which offer greater fitness become more frequent in the population. Individuals DON T Evolve!!!!! Variation Natural selection requires a source of variation within the population. o There have to be differences o Some individuals must be more fit than others. Mutation Mutation creates variation. o New mutations are constantly appearing. Mutation changes DNA sequence. o Changes amino acid sequence? o Changes protein Change structure? Change function? o Changes in protein may change phenotype & fitness. Sex Sex spreads variation. o One ancestor can have lots of descendants. o Sex causes recombination. o Offspring have new combinations of traits = new phenotypes. Sexual reproduction recombines alleles into new arrangements in every offspring. 1

8 Changes in populations Evolution of populations is really measuring changes in allele frequency All the genes & alleles in a population = gene pool o Factors that alter allele frequencies in a population Natural Selection o Genetic drift Founder effect Bottleneck effect o Gene flow Natural selection Natural selection adapts a population to its environment. o A changing environment. Climate change Food source availability New predators or diseases o Combinations of alleles that provide fitness increase in the population. Genetic Drift Changes in gene frequencies from 1 generation to another because of chance events o Examples: 1 family has a lot of children & grandchildren Therefore has a greater impact on the genes in the population than other families A small group splinters off & starts a new colony = founder effect Famine reduces population to small number & then population recovers & expands = bottleneck Founder Effect When a new population is started by only a few individuals o Some rare alleles may be at high frequency; others may be missing o Skew the gene pool of the new population Human populations that started from small group of colonists Example: white people! Distribution of Blood Types South & Central American Indians were nearly 100% type O for the ABO blood system. Since nothing in nature seems to strongly select for or against this trait, it is likely that most of these people are descendants of a small band of closely related "founders" who also shared this blood type. 2

9 Bottleneck Effect When larger population is drastically reduced by a disaster. o Loss of Variation By chance, some alleles may be overrepresented & others underrepresented among survivors. Some alleles may be eliminated altogether. Narrows the gene pool. Cheetahs o All cheetahs share a small number of alleles less than 1% diversity as if all cheetahs are identical twins o 2 bottlenecks 10,000 years ago o Ice Age last 100 years o poaching & loss of habitat Conservation Issues o Bottlenecking is an important concept in conservation biology of endangered species. Populations that have suffered bottleneck incidents have lost at least some alleles from the gene pool. This reduces individual variation & adaptability. At risk populations. Gene Flow Have a population spread over a large geographic area Individuals can move from one area to another Sub-populations may have different allele frequencies Migrations cause mixing across regions = gene flow o New alleles are moving into gene pool o Reduce differences between populations World Wide Travel o Gene flow in human populations is increasing today o transferring alleles between populations Gene Flow & Human Evolution Are we moving towards a blended world? 3

10 Chapter 23: Population Genetics Essential Questions How can we measure evolutionary change in a population? What produces the variation that makes evolution possible? What are the primary mechanisms of adaptive evolution? Population genetics provides a foundation for studying evolution. Smallest unit of evolution Individuals are selected. Populations evolve. Modern Synthesis Evolution since Darwin Comprehensive theory of evolution took form in early 1940s. Integration of natural selection & Mendelian inheritance (genetics). o aka Neo-Darwinism o R.A. Fisher o J.B.S. Haldane o Theodosius Dobzhansky o Ernst Mayr o Sewall Wright o George Gaylord Simpson o Ledyard Stebbins Populations & Gene pools Concepts o A population is a localized group of interbreeding individuals o Gene pool is collection of alleles in the population Remember difference between alleles & genes! Allele frequency is frequency of allele in a population o How many A vs. a in whole population Evolution of Populations Evolution implies a change in allele frequencies in a population o Hypothetical: what would it be like if allele frequencies didn t change? o Non-evolving population Very large population size (no genetic drift) No migration (in or out) No mutation Random mating (no competition) No natural selection Hardy-Weinberg Equilibrium Hypothetical, non-evolving population o Preserves allele frequencies Serves as a Model o Natural populations rarely in H-W equilibrium o Useful model to measure if forces are acting on a population. 1

11 Hardy-Weinberg theorem Alleles o frequency of dominant allele = p o frequency of recessive allele = q frequencies must add to 100%, so: p + q = 1 Individuals o frequency of homozygous dominant = p 2 o frequency of homozygous recessive = q 2 o frequency of heterozygotes = 2pq frequencies must add to 100%, so: p 2 + 2pq + q 2 = 1 Calculating Frequency of Alleles Example: A wildflower population with 2 flower colors. o Allele for red flower color (R) is completely dominant to the allele for white flowers (r). Population of 500 plants o What is the allele frequency? o What % of gene pool is red allele vs. white allele? o Remember diploid = 1000 alleles R r RR: 320 x 2 = 640 rr: 20 x 2 = 40 Rr: 160 x 1 =160 Rr: 160 x 1 =160 R = 800/1000 = 80% r = 200/1000 = 20% p = 0.8 q = 0.2 Application of HW theorem What is the frequency of an allele in the population o Example: What % of the human population carries allele for PKU (phenylketonuria)? o ~ 1 in 10,000 babies born in the US is born with PKU, which results in mental retardation & other problems if untreated. o Disease is caused by a recessive allele. PKU o frequency of homozygous recessive individuals (q 2 ) = 1 in 10,000 or o frequency of recessive allele (q): q = 0.01 o frequency of dominant allele (p): p = 1 q = 0.99 o frequency of carriers, heterozygotes (2pq): 2 x (0.99 x 0.01) = or ~2% ~2% of the US population carries the PKU allele 2

12 Implications of HW theorem In H-W population, all alleles remain at the same frequencies. If allele frequencies change, then population is not in equilibrium & evolution is occurring. Population biologists measure & study. o Sampling of individuals & genetic testing. o Measure from year to year. Using H-W Theorem Microevolution o generation to generation change in a population s allele frequencies. Measuring changes in population from generation to generation. Mutation & sexual recombination produce the variation that makes evolution possible. Mutation Mutation creates variation. o New genes & new alleles originate only by mutation o Only mutations to sex cells can be passed on Mutation changes DNA sequence. o Changes amino acid sequence o Changes protein Change structure? Change function? o Changes in protein may change phenotype & therefore change fitness. Most mutations are deleterious Types of Mutations Point mutations o sickle cell anemia Duplications o hemoglobin chains, fetal hemoglobin o olfactory receptors o immunoglobulins o trnas & rrnas Rearrangements o Translocations Sexual Recombination Sex spreads variation o Sex causes recombination segregation & independent assortment o Offspring have new combinations of traits = new phenotypes Sexual reproduction recombines alleles into new arrangements in every offspring. Selection & Variation Natural selection requires a source of variation within the population. o There have to be differences. o Some individuals are more fit than others. Genetic variation is the substrate for natural selection. 3

13 Types of Selection The effect of selection depending on what is fit Directional Selection Environment favors one extreme. Directional selection for beak size in Galápagos population of medium ground finch: Drier years = thicker shelled seeds = select stronger billed birds. Directional selection for beak size in Galápagos population of medium ground finch: Drier years = thicker shelled seeds = select stronger billed birds. 4

14 Variation Discrete vs. Quantitative Characters o red vs. white flower color = discrete o Human height = quantitative Polymorphic o Morphs Distinct types in a population Geographic variation o Clines Polymorphic Clines Plant height varies with altitude, but still same population Preserving Variation Diploidy o Genetic variation even lethal alleles are hidden in heterozygotes Balancing Selection o Balanced polymorphism Maintaining 2 or more phenotypes through selection Heterozygote advantage Frequency-dependent selection 5

15 Heterozygote Advantage Heterozygotes have a greater fitness o Maintain both alleles in population o Sickle cell anemia Heterozygotes are protected severest effects of malaria & do not develop sickle cell disease. Frequency-dependent Selection Fitness of any morph decrease if it becomes too common. Selection against more abundant phenotype. Consider action of both predators & parasites. Sexual Selection Natural selection for mating success. o Competition amongst males for females. Ritual displays & battles between males. o Female choice Courtship displays to attract females. Female choice rules animal kingdom! Males may go to extremes. (Sexual dimorphism) Limitations of Natural Selection Natural selection cannot fashion perfect organisms. o Evolution is limited by genetic constraints. legacy of ancestral genes existing variations may not be ideal o Adaptations are often compromises. Adaptation for one situation may be limitation for another. o Chance & natural selection interact. The founders may not be the fittest 6

16 Chapter 24: The Origin of Species Essential Questions How and why do new species originate? What is a species? Biological species concept o Defined by Ernst Mayr o Population whose members can interbreed & produce viable, fertile offspring. o Reproductively compatible Biological Species Concept Similar body & colorations, but are distinct biological species because their songs & other behaviors are different enough to prevent interbreeding. Diversity & Taxonomy The Tree of Life o Species are the smallest unit at the ends of branches. o Basic unit for organizing & categorizing living things. o Smallest unit by which we measure diversity. Reproductive Isolation Species concept hinges on reproductive isolation. Biological barriers that impede members from producing viable offspring. Before vs. after fertilization Pre-zygotic barriers (before the zygote). Post-zygotic barriers (after the zygote). Prezygotic Barriers Impede mating or hinder fertilization if mating occurs. Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic isolation 1

17 Habitat Isolation Two species may occupy different habitats within same area so may encounter each other rarely. 2 species of garter snake, Thamnophis, occur in same area, but one lives in water & the other is terrestrial. Temporal Isolation Species that breed during different times of day, different seasons, or different years cannot mix gametes. Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer. Behavioral Isolation Courtship rituals that attract mates & other unique behaviors to a species are effective reproductive barriers. Blue footed boobies mate only after a courtship display unique to their species. Mechanical Isolation Morphological differences can prevent successful mating. Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen. For many insects, male & female sex organs of closely related species do not fit together, preventing sperm transfer. Lack of fit between sexual organs: hard to imagine for us, but a big issue for insects with different shaped genitals! Gametic Isolation Sperm of 1 species may not be able to fertilize eggs of another species. o Variety of mechanisms. Chemical incompatibility. Sperm cannot survive in female reproductive tract. Biochemical barrier so sperm cannot penetrate egg. Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species red & purple are unable to fuse. 2

18 Postzygotic Barriers Prevent hybrid zygote from developing into a viable, fertile adult. o Reduced hybrid viability o Reduced hybrid fertility o Hybrid breakdown Reduced Hybrid Viability Genes of different parent species may interact & impair the hybrid s development. Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail. Reduced Hybrid Fertility Even if hybrids are vigorous they may be sterile Chromosomes of parents may differ in number or structure & meiosis in hybrids may fail to produce normal gametes Habitat Breakdown Hybrids may be fertile & viable in first generation, but when they mate offspring are feeble or sterile. Strains of cultivated rice have accumulated recessive alleles. Hybrids are vigorous but plants in next generation are small & sterile. They are on a path to separate species. Speciation Species are created by a series of evolutionary processes. o Populations become isolated reproductively isolated geographically isolated o Isolated populations evolve independently. Isolation o Allopatric o Sympatric 3

19 Allopatric Speciation Allopatric = other country o geographic separation migration physical barrier Harris s antelope squirrel inhabits the canyon s south rim (L). Just a few miles away on the north rim (R) lives the closely related white tailed antelope squirrel Sympatric Speciation Sympatric = same country o Isolation even though members of population remain in contact o What causes this isolation? Chromosomal changes polyploidy o mostly in plants o oats, cotton, potatoes, tobacco, wheat Non-random mating Adaptive Radiation Evolution of many diversely adapted species when introduced to various new environmental challenges & opportunities. Adaptive Radiation Many ecological niches open Evolution of many diversely-adapted species from a common ancestor to fill niches o Darwin s finches o Mammals Review Speciation is a process. Populations become isolated o Geographic isolation different environmental conditions: food, predators, disease, habitat different selection pressures genetic drift o Reproductive isolation different selection pressures: sexual selection Isolated populations evolve independently 4

20 Current Debate Does speciation happen gradually or rapidly perhaps in response to environmental change o Gradualism Charles Darwin Charles Lyell o Punctuated equilibrium Stephen Jay Gould Niles Eldredge Gradualism Gradual divergence over long spans of time. o Assume that big changes occur as the accumulation of many small ones. Punctuated Equilibrium Rate of speciation is not constant Species undergo most change when they 1st bud from parent population As separate species, remain static for long periods of time Stephen Jay Gould ( ) Harvard paleontologist & evolutionary biologist o Punctuated equilibrium. o Prolific author. Popularized evolutionary thought. 5

21 Evolution is NOT Goal-Oriented An evolutionary trend does not mean that evolution is goal oriented. The modern horse is the only surviving twig of an evolutionary bush with many divergent trends. Convergent Evolution Flight evolved 3 separate times. Evolving similar solutions to similar problems. Parallel Evolution Parallel paths. Filling similar niches therefore exhibit similar adaptations. But are not closely related. Co-Evolution Predator-prey relationships Parasite-host relationships Flowers & pollinators Darwin Awards Named in honor of Charles Darwin, the father of evolution, the Darwin Awards commemorate those who improve our gene pool by removing themselves from it. The Darwin Awards salute the improvement of the human genome by honoring those who accidentally kill themselves in really stupid ways. Of necessity, this honor is generally bestowed posthumously. 6